1. Field of the Invention
The present invention relates to an exhaust emission control device for an internal combustion engine that includes a NOx cleaning catalyst for reducing trapped NOx under a reducing atmosphere in an exhaust passage thereof, the exhaust emission control device supplying a reducing agent to an upstream side of the NOx cleaning catalyst, so as to cause the NOx cleaning catalyst to perform a NOx reducing operation, a method of controlling the exhaust emission control device, and an engine control unit.
2. Description of the Related Art
Conventionally, as an exhaust emission control device of this kind, one disclosed in the publication of Japanese Patent Publication No. 2692380 is known. The exhaust emission control device shown in FIG. 1 in this publication is applied to a gasoline engine. The gasoline engine has an exhaust passage provided with a NOx cleaning catalyst. The NOx cleaning catalyst traps NOx (nitrogen oxides) in exhaust gases under an oxidizing atmosphere, and reduces the trapped NOx when exhaust gases under a reducing atmosphere are supplied thereto, to thereby clean the exhaust gases, i.e. reduce exhaust emissions. Further, the exhaust emission control device is disposed in the exhaust passage at a location downstream of the NOx cleaning catalyst, and is provided with an air-fuel ratio sensor for detecting an air-fuel ratio of exhaust gases.
As shown in FIGS. 12 and 13 of the publication, in the exhaust emission control device, during a lean burn operation of the engine, an operation time period T of the lean burn operation is counted, and when the operation time period T exceeds a predetermined value To, reduction control is carried out so as to reduce NOx trapped in the NOx cleaning catalyst. More specifically, by switching the engine from the lean burn operation to a rich burn operation, exhaust gases under the reducing atmosphere, containing unburned fuel as a reducing agent, are supplied to the NOx cleaning catalyst. During the reduction control, when the air-fuel ratio detected by the air-fuel ratio sensor changes over from a lean air-fuel ratio to a rich air-fuel ratio, presumably, all the NOx trapped in the NOx cleaning catalyst has been reduced, and hence it is judged that the reduction control should be terminated, so that the reduction control is terminated to switch the engine from the rich burn operation to the lean burn operation.
Further, there has been recently proposed a technique of applying an auxiliary catalyst provided with an oxygen storage capacity to the NOx cleaning catalyst so as to increase the NOx purifying capability of the NOx cleaning catalyst. As an auxiliary catalyst of this kind is used a ceria-zirconia composite oxide disclosed e.g. in Japanese Laid-Open Patent Publication (Kokai) No. H11-165067. If such an auxiliary catalyst is used, when exhaust gases under the reducing atmosphere are supplied to the NOx cleaning catalyst, the auxiliary catalyst absorbs oxygen, whereby an operation for reducing NOx trapped in the NOx cleaning catalyst is accelerated, whereas when exhaust gases under the oxidizing atmosphere are supplied to the NOx cleaning catalyst, the auxiliary catalyst releases oxygen, whereby a NOx trapping operation of the NOx cleaning catalyst is accelerated.
To increase the NOx purifying capability of the NOx cleaning catalyst, if the auxiliary catalyst disclosed in Japanese Laid-Open Patent Publication (Kokai) No. H11-165067 is applied to the exhaust emission control device disclosed in the publication of Japanese Patent Publication No. 2692380, there can arise the following problems: In the case of the auxiliary catalyst disclosed in Japanese Laid-Open Patent Publication (Kokai) No. H11-165067, as shown in FIG. 3 in Japanese Laid-Open Patent Publication (Kokai) No. H11-165067, when the temperature of the auxiliary catalyst is within a predetermined temperature range (temperature range between 400 and 750° C.), the auxiliary catalyst has a very high oxygen storage capacity, whereas when the temperature of the auxiliary catalyst is not within the predetermined temperature range, the auxiliary catalyst has only a fairly low oxygen storage capacity. Therefore, during execution of the above-described reduction control, when the temperature of the NOx cleaning catalyst is not within the predetermined temperature range mentioned above, due to the fairly low oxygen storage capacity of the auxiliary catalyst, the reliability of the air-fuel ratio detected by the air-fuel ratio sensor is lowered, which make it impossible to terminate the reduction control in proper timing. As a result, when a time period over which the reduction control is executed is too short, the degree of reduction of NOx trapped in the NOx cleaning catalyst is lowered to degrade the NOx purifying capability of the NOx cleaning catalyst, resulting in an increase in exhaust emissions. On the other hand, when the time period over which the reduction control is executed is longer than required, fuel economy is degraded.